1. Field of Invention
The present invention relates to optically addressable spatial light modulators and to a method and apparatus for driving optically addressable spatial light modulators. The present invention also relates to holographic displays comprising optically addressable spatial light modulators.
2. Description of the Art
It is well known that a three-dimensional image may be presented by forming an interference pattern or hologram on a planar surface. The three-dimensional image is visible when the hologram is appropriately illuminated. Recently, interest has grown in so-called computer generated holograms (CGHs) which offer the possibility of displaying high quality images, which need not be based upon real objects, with appropriate depth cues and without the need for viewing goggles. Interest is perhaps most intense in the medical and design fields where the need for realistic visualisation techniques is great. Typically, a computer generated hologram involves the generation of a matrix of data values (each data value corresponding to a light transmission level) which simulates the hologram which might otherwise be formed on a real planar surface. An important component of the CGH system is an optically addressed spatial light modulator (OASLM). This device comprises a photosensor layer, typically silicon (Si), and a liquid crystal (LC) layer with electrodes by which a voltage is applied across both layers. When a pattern of light of appropriate wavelength is incident onto the photosensor its resistance falls and allows most of the applied voltage to drop across the LC which is then switched in those areas which are illuminated A light beam incident on the front of the LC layer is then modulated by the LC and reflected from the Si (or from an incorporated mirror) in such a way that an image is presented to a viewer.
A holographic display system employing CGHs is described in GB2330471A and is further illustrated in FIG. 1. The illustrated approach is known as Active Tiling™, and involves the use of a relatively small Electrically Addressable Spatial Light Modulator (EASLM) 1 in combination with a relatively large Optically Addressable Spatial Light Modulator (OASLM) 2. The holographic matrix (CGH) is subdivided into a set of sub-holograms, with the data for each sub-hologram being passed in turn to the EASLM 1. The EASLM 1 is illuminated from one side with incoherent light 3. The OASLM 2 comprises a sheet of liquid crystal (in one example the liquid crystal is a bistable ferroelectric liquid crystal) which is switched from a first to a second state by a suitable voltage in the presence of incident light In one method, Guide optics 4, which may include a shutter, disposed between the EASIM 1 and the OASLM 2, cause the output of the BASLM 1 (i.e. light transmitted through the EASIM 1) to be stepped across the surface of the OASLM 2. The bistable nature of the OASLM liquid crystal means that the portion or “tile” 5 of the OASLM 2 onto which a sub-holographic image is projected and appropriate switching voltage applied, remembers  that image until such time as the OASLM is reset by the application of an electrical voltage. It will be appreciated that, when all of the component images have been written to the OASLM, the OASLM will have “stored” in it a replica of the complete holographic matrix. The image may be replayed by a coherent beam incident 6 on the LC. When the next holographic image is ready to be transferred to the OASLM a reset voltage is applied to remove the existing image from the OALSM and prepare it for loading the next image. The holographic display also typically comprises a large output lens, although this is not shown in FIG. 1.
The structure of a typical OASLM is illustrated in FIG. 2 and a simplified equivalent circuit is shown in FIG. 3. In this circuit the voltage is applied to the Si side of the device and the LC side is earthed. This convention will be followed throughout the following discussion. From left to right in FIG. 2, the layers are as follows; a first glass layer 1, an indium tin oxide layer 2 which forms a first transparent electrode, a silicon photosensor layer 3, a light blocking layer 4, a mirror 5, a first alignment layer 6 which may be formed by brushing a polyimide layer, a liquid crystal (LC) layer 7, a second alignment layer 8, a second indium tin oxide layer 9, and a second glass layer 10. A voltage source 11 is coupled to the two indium tin oxide layers 2, 9 in order to control the switching of the OASLM. The silicon-indium tin oxide layer junction acts as a diode; when a voltage of a first positive polarity is applied across the device this diode is forward biased and most of the voltage will be dropped across the LC layer 7, whilst when a voltage of a second, negative polarity is applied across the device, most of the voltage will be dropped across the silicon layer 3 unless write light is applied in which case the voltage will be dropped across the LC layer 7. The bias of the second polarity is referred to as the “photosensitive direction”. When the bias is in the photosensitive direction and with no illumination, the voltage appearing across the LC layer 7, Vlc, is given by the capacitive division of the total voltage appearing across the OASLM:Vlc=CSi/(Clc+CSi),where CSi and Clc are the capacitances of the silicon and LC layers respectively. As charge is generated in the Si layer, so the voltage across the LC rises.
In the ideal case a Schottky barrier is formed in the OALSM by the silicon and indium-tin-oxide (ITO) transparent electrode. This gives behaviour some way between that of a photodiode and a photoconductor. If ohmic contacts are made then photoconductor behaviour results. The major problem with a pure photoconductor is the dark leakage current which is not sufficiently low to keep the voltage from dropping across the LC in a non-illuminated addressed state. A photodiode requires the deposition of p-doped, intrinsic and n-doped Si and is a complicated process. For a photodiode under reverse bias, when a photon is absorbed to produce an electron-hole pair in the Si, the hole and electron are separated and drift to the contacts. The blocking contacts stop the carriers so that once they are collected the response is complete. The photocurrent varies linearly with the light intensity over a wide range of intensities because one electron-hole pair is collected for each absorbed photon. With the application of a positive applied voltage the photodiode is forward biased so that all of the voltage should drop across the LC. The presence of a write light should not affect the state of the LC significantly, with a positive voltage applied. When a negative applied voltage is applied, the photodiode is reverse biased, blocking the current, so that ideally the voltage across the LC is unchanged. When a write light illuminates the photodiode a photocurrent charges the LC to a negative voltage and causes switching. This voltage is maintained across the LC until the drive voltage goes positive again.
For more details of the operating theory of spatial light modulators see, for example, “Spatial Light Modulator Technology, Materials, Devices and Applications”, edited by U Efron, published by Marcel Dekker Inc. 1995 and “Optimisation of ferroelectric liquid crystal optically addressed spatial light modulator performance”, Perennes F & Crossland W A, Opt. Eng. 36 (8) 2294-2301 (August 1997).
A typical voltage signal for controlling an OASLM is illustrated in FIG. 4. Prior to each write phase, a blanking pulse is applied to the device. The blanking pulse has a relatively large amplitude and duration and has a polarity in the opposite direction to the polarity of the photosensitive direction (i.e. positive). Typically, the whole device may be illuminated entirely with write light for the duration of the blanking pulse. The blanking pulse results in the molecules of the liquid crystal layer being oriented in a first direction. Each blanking pulse is followed by a write pulse which has a polarity in the photosensitive direction, the write pulse may be separated in time from the blanking pulse or may follow it immediately. There will usually be some means of DC balancing the blanking pulse and write pulse so that there is net zero DC, but this is not shown in the Figure. During a write pulse, the silicon side of the device is illuminated with the desired pattern. The result is that the applied voltage is dropped across the liquid crystal layer in those regions where the device is illuminated, causing the liquid crystal to switch to a second state. In non-illuminated regions of the device, the liquid crystal does not switch. The “switched” pattern in the liquid crystal is used to modulate a light beam incident on the liquid crystal side of the device.
Prior art drive waveforms are described in, for example “Optimisation of ferroelectric liquid crystal optically addressed spatial light modulator performance”, Perennes F & Crossland W A, Opt. Eng. 36 (8) 2294-2301 (August 1997). In this example the erase pulse is immediately followed by a write pulse. This appears to be a simple bipolar pulse but it must be remembered that in fact it is two adjacent monopolar pulses of different polarity and different function, which may be separated in time. Applied Optics Vol. 31, No. 32, pp. 6859-6868, 10 Nov. 1992, describes both erase and write pulses which are bipolar. These may be applied with either polarity, i.e. leading part positive, training part negative, or vice versa, since the device used has ohmic contacts. The function of the bipolar pulse in this example is to maintain DC balance.
Whilst a write pulse having a polarity in the photosensitive direction will be most effective at switching illuminated areas of a device, there will be a tendency for non-illuminated areas to switch since they will receive a reduced voltage of the same polarity. Careful design of the write pulse (shape, amplitude and width) is therefore required in order to achieve maximum discrimination between switching of illuminated and non-illuminated regions. In addition, it is desirable to maximise the region of pulse amplitude-width space in which the device is operated in order to compensate for variations within the device, e.g. cell spacing, and in operating conditions, e.g. temperature. Furthermore it is desirable to maximise switching speed (minimise switching pulse width) in order that images may be updated rapidly, e.g. for frame sequential colour. The relative thicknesses of Si and LC layers also influence switching since this changes their capacitance and thus the proportion of voltage appearing across the LC due to capacitive division of voltage. These parameters will affect the switching characteristic of the OALSM although not that of the LC. All of these parameters need to be optimised.